Passive optical networks (PONs) have the ability to provide high levels of bandwidth while only requiring low energy consumption. This makes PONs useful in providing broadband access. In addition, PONs provide large, huge capacity, small attenuation, low operational expenditures, and longevity. PONs consume less energy and are more efficient than competing technologies such as other types of optical and wireless systems.
PONs are more energy efficient per bit than hybrid fiber-copper based access technologies, e.g., fiber-to-the-node (FTTN), and wireless access solutions, e.g., WiMAX. PONs are also more energy efficient than fiber-to-the-home (FTTH) network technologies such as point-to-point and active optical access networks. These qualities make PON deployments attractive especially to address concerns about the green-house impact of the Internet.
FIG. 1 is a diagram of a conventional tree-based PON. Typically, PONs have a physical tree topology with the optical line terminal (OLT) located at the root and the subscribers connected to the leaf nodes of the tree at a distance of up to 20 km. The PON connects the OLT to multiple optical network units (ONUs), also known as optical network terminals (ONTs) or customer premises equipment. The ONUs are connected to the OLT through a 1:N optical splitter/combiner. To protect the communication between the OLT and the ONU, a second protection fiber (shown as dashed lines) duplicates the connections and links of the PON significantly increasing the cost. This protection fiber is used when the primary fiber fails.
In the point-to-multipoint downstream direction (i.e., OLT to ONU), the OLT is able to broadcast data to all ONUs simultaneously due to the directional property of the optical splitter/combiner. In the upstream direction, however, ONUs cannot communicate directly with one another. Instead, each ONU is able to send data only to the OLT in a multipoint-to-point manner. To allow all ONUs to share the optical line between the OLT and the ONUs, a multiple access protocol is utilized, such as time division multiple access (TDMA) is utilized.
In both Institute of Electrical and Electronics Engineers (IEEE) Ethernet PON (EPON) and International Telecommunication Union Standardization Sector (ITU-T) Gigabit PON (GPON), a polling mechanism is proposed to facilitate bandwidth allocation. More specifically, each ONU reports its required bandwidth (i.e., queue occupancy) to the OLT and the OLT informs the ONUs about their assigned upstream transmission windows in the downstream frame. EPON introduces report and gate messages in both the upstream and downstream directions to report and specify the ONU upstream transmission grants, respectively. In GPON, each upstream and downstream frame contains a dynamic bandwidth report (DBRu) and a physical control block (PCBd), respectively. DBRu is used for reporting the required bandwidth by an ONU. PCBd includes a bandwidth map (BWmap) field to specify the ONU upstream transmission grants.